Laser phase noise is an important metric that can drive the performance of systems ranging from instrumentation to communication systems. Opto-electronic systems reducing laser phase noise are an ongoing development topic. In particular, achieving the performance of high performance bench top lasers by cleaning up the output of cheap and compact semiconductor lasers is a very important goal. High power semiconductor lasers (above a few tens of mW) tend to have severely degraded linewidth. The linewidth broadening of high power distributed feedback (DFB) lasers is for example typically a result of laser phase noise, so that cleaning laser phase noise could lead to a dramatic improvement of high power semiconductor laser linewidth. Laser phase noise is typically a limiting factor in instrumentation, and can be a limiting factor in long haul communication due to fiber dispersion. With the increasing importance of phase encoding in optical communications, laser phase noise also increasingly becomes problematic in the absence of fiber dispersion since laser phase noise is directly converted into amplitude noise at the receiver optics. Such encoding schemes include binary phase shift keying, differential phase shift keying, quadrature phase shift keying, as well as homodyne or heterodyne (coherent) receivers. On-chip resonators with quality factors (Q-factors) on the order of 100 million have also been demonstrated (see D. K. Armani, T. J. Kippenberg, S. M. Spillane, K. K. Vahala, “Ultra-high-Q toroid microcavity on a chip”, Nature 421, 925-928 (2003), hereby incorporated by reference in its entirety.) These allow, for example, on-chip generation of frequency combs (thus also acting as a multi-wavelength light source), or can be used to generate ultra-low noise RF reference signals (see D. Eliyahu, D. Seidel, L. Maleki, “Phase noise of a high performance OEO and an ultra low noise floor cross-correlation microwave photonic homodyne system”, Proceedings of Frequency Control Symposium, 2008 I EEE International, Honolulu, Hi. pp. 811-814, 19-21 May 2008. ISBN: 978-1-4244-1794-0 DOI: 10.1109FREQ.2008.4673111, hereby incorporated by reference in its entirety). In order to couple to resonators with such ultra-high quality factors, a light source with correspondingly small line width has also to be used. Since using a bulky and expensive laser defeats the point of using an integrated resonator, the ability to clean up a high power semiconductor laser is particularly powerful in this context.
As a first building block for laser phase noise removal, a laser phase noise detection scheme is required. Typically, an imbalanced interferometer is used, where one of the arms of the interferometer contains a delay line. When the interferometer is properly biased (at the 3 dB point) this results in an output signal whose power corresponds to the derivative of the phase noise, for slowly varying phase noise. Precisely, for a time delay τ and a time varying phase noise φ, the output power of the interferometer is proportional to (1+ sin(φ(t)−φ(t−τ)))/2 which is approximately (1+τ×dφ/dt)/2 for a slowly varying phase, ω denotes the angular frequency of a sinusoidal phase perturbation, i.e. φ=φ0 sin(ωt). The output of the imbalanced interferometer is then ½+ sin 2φ0 sin(ωτ/2)cos(φt−ωτ/2))/2, i.e. the small signal laser phase noise to modulator output power modulation transfer function is given by sin(ωτ/2). This is the conversion gain of the imbalanced interferometer optical phase noise detection scheme. For slowly varying phase noise, increasing the delay line also increases the sensitivity of the noise detection scheme. However there are high frequency zeroes in the transfer function corresponding to ωτ/2=π+N×π, wherein N is an integer. Increasing the delay line also pulls in the high-frequency roll off towards lower frequencies, leading to a sensitivity/optical bandwidth tradeoff. Also, since at low frequencies the interferometer essentially applies a derivative to the phase noise, its output is typically followed by an integrator. This way the transfer function zero at ω=0 is compensated for since the aggregate electro-optic transfer function becomes sin(ωτ/2)/ω.
Phase noise reduction is also described in 3. M. Bagheri, F. Aflatouni, A. Imani, A. Goel, H. Hashemi, “Semiconductor laser phase-noise cancellation using an electrical feed-forward scheme”, Opt. Lett. 34, 2979-2981 (2009), hereby incorporated by reference in its entirety.